1. Field of the Invention
The present invention relates to an A/D converter and, more particularly, it relates to a sequential comparison type A/D converter.
2. Description of the Prior Art
A conventional type A/D converter includes a D/A converter which generates a reference voltage; a comparator which compares a sampled analog input voltage with the reference voltage; and a sequential comparator control circuit which repeats the comparison steps that input a digital value into the D/A converter in accordance with the result of the determination by the comparator so as to obtain the result of the conversion. The comparator receives the analog signal input to be converted at its first input, to be compared therewith and receives the analog signal input from the D/A converter at its second input. These signals received at the first input and the second input are compared in the comparator and the compared output is supplied to a sequential comparator control circuit. In a sequential comparator control circuit, the digital output value is varied in accordance with the output of the comparator, so that digital signals are supplied to the D/A converter, which converts the digital signals to an analog signal. The analog signal is supplied to the second input of the comparator, which carries out a second comparison. As a result, comparisons are sequentially carried out at a first time, a second time, and so on, until the first input and the second input are equal to each other, at which point the digital signal corresponding to the analog signal value is obtained.
In the sequential comparison system discussed above, the comparison is started from one half of the maximum reference voltage usually generated in the D/A converter, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-160219. However, when the analog input voltage varies over a wide range including values at which a malfunction can occur in the comparator, as explained in detail hereinafter.
If the comparison is started from a 3/4 reference volage, the above-mentioned malfunction is eliminated, as disclosed in Japanese Unexamined Patent Publication No. 58-53222. However, with this later method, when the next value is set in accordance with the comparison result, an error in comparison may routinely occur, as explained in detail hereinafter.